1. Field of the Invention
Apparatuses consistent with the present invention relate to a diffuser with a shape profile for reducing a speckle noise, and a projection system which reduces the speckle noise by employing the diffuser.
2. Description of the Related Art
Projection systems display a color image by modulating light having a plurality of colors corresponding to image information, and extending and projecting the light on a screen. Various kinds of light sources can be used in projection systems. In particular, a laser light source may be preferable, since the laser light source has a high intensity of light and brightness in order to provide a clear image having a high contrast. However, a laser beam of the laser light source is highly coherent, which causes speckle noise. Speckle is a mottled pattern generated by an inherent wave property of a laser beam that is incident on a screen, due to the roughness of a surface of the screen Speckle significantly reduces the image quality.
In order to reduce the speckle noise, an image having N number of uncorrelated speckle patterns should be formed. A speckle contrast C is defined in relation to a dispersion σI with respect to a mean of the intensity of light <I> due to a speckle (C=σI/<I>). Accordingly, when the N number of uncorrelated speckle patterns are formed, the speckle contrast is reduced by 1/N1/2 times. Accordingly, it is very important to form the N number of uncorrelated speckle patterns. To do so, various methods have been suggested, such as producing a spatial diversity of laser beam, producing a time diversity of laser beam, enhancing light bandwidth, and performing a phase de-correlation.
An example of the phase de-correlation method is a method in which a driving diffuser is disposed on an image plane. A fine structure of the surface of the driving diffuser may be expressed by a correlation length rc. When the driving diffuser moves by at least the correlation length rc, the uncorrelated speckle patterns are formed. In order to efficiently reduce the speckle, the movement distance of the driving diffuser should be several times the correlation length rc. The ideal case for reducing the speckle is the case when the driving diffuser has a small correlation length rc. In particular, in a scanning projection system having a short pixel lighting time, a movement velocity of the diffuser required for reducing the speckle can be reduced by minimizing the correlation length rc. On the other hand, since a scattering angle is proportional to λ/rc, the minimized correlation length rc, increases light scattering on the driving diffuser, which causes an optical loss. Thus, to avoid this loss, an aperture of a projection optical system should be increased. Accordingly, when a compact optical system is designed, there is a limit to simultaneously realize a small aperture size and reduce speckle noise.